EARLY PALEOZOIC POST-BREAKUP MAGMATISM ALONG THE NORTHERN CORDILLERAN MARGIN: NEW GEOCHRONOLOGICAL AND GEOCHEMICAL RESULTS FROM THE KECHIKA GROUP, SOUTH-CENTRAL YUKON

An unresolved problem in Cordilleran geology concerns the timing and tectonic significance of post-breakup volcanic rocks that crop out within passive margin successions from NW Canada to the SW United States. Simplistic rifting models for the Cordilleran margin, including pure- and simple-shear rift scenarios, do not fully explain the relationship between early Cambrian lithospheric breakup and late Cambrian to Ordovician volcanism. In south-central Yukon, lower Paleozoic alkaline rocks of the Kechika group record post-breakup tectonism along the northern Cordilleran margin. This study integrates new field and laboratory data to constrain the physical volcanology, timing, and mantle-crustal sources of Kechika group rocks and test comparisons within modern rifted margins.

Kechika group lithofacies are indicative of submarine volcanic centers and include pillow lava, sediment-matrix basalt breccia, and monomictic basalt breccia. Associated mafic sills are likely comagmatic sediment-sill complexes. New CA-TIMS zircon U-Pb results have identified at least two episodes of mafic magmatism during the latest Cambrian and Early Ordovician, respectively. Kechika group lavas were generated by the low degree partial melting of an enriched mantle source as indicated by high Nb/Y and Ti/V, OIB-like geochemical signatures, and juvenile to intermediate Nd-Hf isotope compositions. Some comagmatic intrusive rocks show broadly similar results with the exception of lower Nb/Th values and negative Nd-Hf isotope compositions that suggest crustal contamination.

The timing of Kechika group deposition is consistent with previous stratigraphic constraints for post-breakup volcanism along the Cordilleran margin. Coeval volcanic rocks, including those of the Menzie Creek and Marmot formations in NW Canada, are spatially associated with both margin-parallel normal faults and inferred transfer-transform zones at high angles to the rifted margin. Potential modern analogues for the Kechika group include syn- to post-breakup strata along the Newfoundland rifted margin, such as those related to the Jurassic-Cretaceous Orphan, Fogo, and Newfoundland Seamounts and post-Albian sills in the Newfoundland basin.